Electrical insulating oil is commonly used in equipment for generating and distributing electrical power. Such equipment includes transformers (sometimes called oil-immersed transformers), tap-changers and circuit breakers. When a fault occurs in such equipment, fault gases may evolve in the insulating oil. Analysis of such fault gases may be used to provide a diagnosis of the operation of the electrical equipment. For example, transformer faults typically include arcing, corona discharge and overheating (pyrolysis) and it has been found that such faults can be diagnosed by analysing the quantities of certain fault gases including ethane, methane, ethylene, acetylene, carbon monoxide, carbon dioxide, hydrogen and/or oxygen that are present in the transformer oil. Such analysis can determine not only what type of faults are present, but also how severe the faults are. Accurate knowledge of the condition of electrical equipment, including transformers, is of paramount importance to electrical utilities as it allows assets to be optimised and potentially expensive failures to be avoided.
Dissolved Gas Analysis (DGA) and moisture measurement of the insulation oil are recognised as the most important tests for the condition assessment of transformers. DGA is also fast becoming the key diagnostic technique for monitoring load tap changers (LTCs).
It will be noted that the term ‘gas’ as used herein may embrace the term ‘vapour’.
One problem associated with DGA is cross contamination between oils. As a result, measurement equipment used to perform DGA on an oil sample from one piece of equipment, say a transformer, cannot readily be used to perform DGA on an oil sample from another piece of equipment, say a tap changer, without extensive cleaning of the equipment between the respective measurements. Cleaning can be time consuming and often has to be performed off site. Alternatively, separate measuring equipment may be provided, but this is costly.
For example, convention measurement equipment tends to use a gas chromatograph to perform DGA. Graph chromatographs are relatively expensive, complex and cumbersome to use. Moreover, the operation of such equipment often requires a relatively high level of training and the processing of oil samples is slow. As a result, analysis using gas chromatograph techniques is normally performed off-site in a laboratory by suitably trained technicians. To compound this problem, gas chromatographs are relatively difficult to clean to the extent that is necessary to avoid cross-contamination.
It would be desirable to provide an apparatus for performing DGA which can measure oil samples from different sources relatively easily while minimising or avoiding cross contamination. It would also be desirable for such an apparatus to lend itself to on-line DGA analysis, i.e. analysis performed on site with the apparatus connected to the equipment, e.g. transformers and/or tap changers, from which the oil samples are extracted.